1. Field of the Invention
The present invention relates to a method for manufacturing polyolefin foam beads, and more particularly to a method for manufacturing polyolefin foam beads continuously.
2. Description of the Related Art
Polymer foams made from polyurethane, polystyrene, or polyolefin are widely used as a shock absorbing material, heat insulating material, and packing material due to their flexibility, durability, light weight, and low thermal conductivity.
Conventionally, the polyolefin foam beads are commonly manufactured by vaporizing a foaming agent in a polyolefin resin through a batch process which is divided into a foaming agent impregnating step followed by a foaming step. Such method of manufacturing polyolefin foam beads is disclosed in U.S. Pat. No. 4,378,859. By referring to FIG. 2, the impregnating and the foaming steps will be described in more detail.
In the impregnating step, a foaming agent, dispersing agent, a resin particles, and water are added and mixed in a high-pressure impregnation chamber 201 under heat to facilitate the impregnation of foaming agent into the resin. Here, to prepare the resin particles for a subsequent foaming method, the impregnating step is carried out at a temperature between the softening point and the melting point of the resin.
In the foaming step, the resin particles containing the foaming agent are discharged from the high-pressure impregnation chamber 201 to a low-pressure foaming chamber 202. At the foaming chamber 202, the foaming agent impregnated in the resin particles is then vaporized to produce foam beads. Here, the excessive foaming agent in the upper space in the impregnation chamber is discharged along with the resin particles and water. The excessive foaming agent is recovered by a foaming agent recovery device 204 comprises of a cooler, a compressor and a layer separator. After being separated from water in the layer separator, the foaming agent is recycled into use and the water is retrieved to a water recovery chamber 203 to be put into use again.
However, the above conventional method has the following drawbacks.
First, the vapor pressure of the foaming agent at the operation temperature is high (10-30 kg/cm2), and above a certain capacity the scale-up of high-pressure vessel is technically difficult, and the manufacturing cost of the pressure vessel increases drastically.
Second, in order to facilitate the mixing of the resin, foaming agent and water in the impregnation chamber, about 30% of the total volume of the impregnation chamber must be left as free gaseous space above the mixture. The amount of excessive foaming agent present as compressed vapor in the free space is about 30-50% of the total foaming agent supplied to the manufacturing method. In order to recover the excessive foaming agent from the expansion chamber, a large recovery device is required.
Third, a large expansion chamber having 20-50 times the volume of the impregnation chamber is required, because the volume of the resin increases by 20-50 times after the foaming step.
Forth, it is. necessary to heat the recovered water by 20-30xc2x0 C. in order to use the water again, because the temperature of the water falls down after the foaming step.
Fifth, in order to discharge all the resin particles in the impregnation chamber, the amount of the resin which can be charged in the impregnation chamber is restricted, and beyond this amount, remained resin particles expand in the impregnation chamber during the foaming step to block the process.
Due to the problems and drawbacks described above, the conventional batch process reveals high capital cost and low production efficiency which yield high price of polyolefin foam to restrict its wide use despite of its desirable properties.
In this connection, it is to be noted that methods for manufacturing foam beads by continuous process are described in U.S. Pat. No. 5,580,503, U.S. Pat. No. 5,753,157, and U.S. Pat. No. 5,928,674. However, the above-mentioned continuous process have the problems that the resin particles block the moving part of pumps and check valves and the foaming agent is apt to be separated from water during the process due to the density difference.
In view of the foregoing, it is an object of the present invention to provide a continuous method for manufacturing polyolefin foam beads with markedly increased efficiency and enhanced productivity to lower the price of the polyolefin foams.
In order to achieve the above object, the present invention provides a method which comprises a impregnating step where resin particles, water and foaming agent are mixed in a impregnation chamber under heat and pressure, followed by a storing step where the resin particles containing the foaming agent are transferred to and stored in a storing chamber. Thereafter, the resin particles are subjected to a foaming step where the particles are discharged continuously or intermittently via a discharging device to produce the foam beads. Optionally, prior to the impregnating step, present method further comprises a supplying step of the resin particles, by which the resin particles are supplied to the impregnation chamber maintained at high pressure. Each step is processed sequentially and repeated continuously by separate chambers.